A split Stirling refrigeration system is shown in FIG. 1. This system includes a reciprocating compressor 14 and a cold finger 16. Two pistons 17 and 18 of the compressor reciprocate in a cylinder 15 in opposition to each other to provide a nearly sinusoidal pressure variation in a pressurized refrigeration gas such as helium. The pressure variation in a head space 19 is transmitted through a supply line 20 to the cold finger 16.
Within the housing of the cold finger 16 a cylindrical displacer 26 is free to move in a reciprocating motion to change the volumes of a warm space 22 and a cold space 24 within the cold finger. The displacer 26 contains a regenerative heat exchanger 28 comprised of several hundred fine-mesh metal screen discs stacked to form a cylindrical matrix. Other regenerators, such as those with stacked balls, are also known. Helium is free to flow through the regenerator between the warm space 22 and the cold space 24. A piston element 30 extends upwardly from the main body of the displacer 26 into a gas spring volume 32 at the warm end of the cold finger.
The refrigeration system of FIG. 1 can be seen as including three isolated volumes of pressurized gas. A working volume of gas comprises the gas in the head space 19 of the compressor, the gas in the supply line 20, and the gas in the spaces 22 and 24 and in the regenerator 28 of the cold finger 16. A second volume is a relatively large dead space in the compressor behind the compressor pistons 17, 18. It is sealed from the working volume by clearance seals between the pistons 17, 18 and the cylinder 15. The third volume of gas is the gas spring volume 32 which is sealed from the working volume by a clearance seal 34 surrounding the drive piston 30. The three isolated volumes of gas are maintained apart by clearance seals. Such seals are imperfect, and therefore allow for slow leakage between these three volumes of gas. This small leakage is beneficial.
Operation of the split Stirling refrigeration system will now be described. At the point in the cycle shown in FIG. 1, the displacer 26 is at the cold end of the cold finger 16 and the compressor is compressing the gas in the working volume. This compressing movement of the compressor pistons 17, 18 causes the pressure in the working volume to rise from a minimum pressure to a maximum pressure and this warms the working volume of gas. The pressure in the gas spring volume 32 is stabilized at a level between the minimum and maximum pressure levels of the working volume. Thus, at some point the increasing pressure in the working volume creates a sufficient pressure difference across the drive piston 30 to overcome retarding forces including a pressure differential across the displacer. The displacer then moves rapidly upward. With this movement of the displacer, high-pressure working gas at about ambient temperature is forced through the regenerator 28 into the cold space 24. The regenerator absorbs heat from the flowing pressurized gas and thereby reduces the temperature of the gas.
The compressor pistons 17, 18 now begin to move away from each other and to expand the working volume. With expansion, the high pressure helium in the cold space 24 is cooled even further. It is this cooling in the cold space 24 which provides the refrigeration for maintaining a temperature gradient of over 200 degrees Kelvin over the length of the regenerator.
At some point in the expanding movement of the pistons 17, 18, the pressure in the working volume drops sufficiently below that in the gas spring volume 32 for the gas pressure differential across the piston portion 30 to overcome retarding forces. The displacer 26 is then driven downward to the starting position of FIG. 1. The cooled gas in the cold space 24 is thus driven through the regenerator to extract heat from the regenerator. The heat added to the regenerator at an earlier time by high pressure working gas is less than the heat subtracted at this time by low pressure working gas. Therefore, there is net refrigeration on the average.
The traditional approach to drive motor design in split Stirling refrigerators has been to utilize a rotary electric drive in the compressor. Lubricated mechanical bearings and linkages are employed to convert rotary motion to oscillating motion. More recently, systems have been developed using a linear electric drive directly coupled to each compressor piston.
Rotary drive systems have the advantages of being lighter and smaller than linear drive systems for given outputs. Also, the rotary drive system is relatively insensitive to loading due to acceleration of the system because major inertial forces are routed through the mechanical bearings. The mechanical bearings also completely define the piston stroke at any speed. On the other hand, rotary drive systems suffer side loads and torsional vibration which can not be easily eliminated. Furthermore, the rolling element bearings, the need for connecting rods with their potential for backlash and the presence of laminated motor windings in the working gas environment have serious negative effects on acoustic noise, induced vibration and generation of gas and debris in the refrigerator.
Linear drive systems have the advantage of the elimination of all rolling element bearings. A single linear gas bearing per piston results in very low acoustic noise generation. Further, excellent dynamic balancing can be achieved by utilizing matched opposed pistons moving colinearly. With the center of symmetry coincident with the center of gravity of the compressor, the result is a very low level of induced vibration. By carrying mechanical loads through short stiff paths, deflections and stresses due to side forces can be kept to a minimum. Also, motor coil windings may be positioned external to the working gas to eliminate any potential of gas contamination due to outgassing. Furthermore, all parts of the windings in a linear motor contribute to the mechanical output. In rotary motors portions of the windings (end turns) are non-axial and do not contribute to the mechanical output although these non-axial portions do contribute to the ohmic losses and to the leakage reactance. These features of linear drive systems result in a very long life and a highly reliable refrigerator.
A disadvantage of the use of linear motors in cryogenic refrigerators is that accelerations of the refrigerator system may cause the compressor pistons to shift in a common direction and thus cause a loss of balance of the system about a center plane. This result is due to the lack of mechanical linkages which otherwise oppose the shifting of the pistons. Other forces such as the force of gravity can also act to pull the pistons in a common direction. For balanced operation, forces on the two pistons must be equal in magnitude but opposite in direction.
Another problem associated with linear motor drives is that angular acceleration and pressure differentials can cause a shift in the mean position of each piston relative to a center plane.
A way to appreciate these two problems is to consider what controls the end points of the stroke of each piston. One solution to the problems of balancing and centering linear drive motors is to use sophisticated feedback control. The coils to the respective drive motors are energized in a fashion which maintains balance while providing the work necessary to compress the refrigerant. Such control systems add undesirable complexity and power requirements to the electronic control of the system.